Collateral - A Gateway Node requires 30,000 ETHO coins compared to the 15,000 ETHO coins needed for a Master Node and 5,000 ETHO coins for a Service Node. This means when upgrading from two Master Nodes, you will likely need to transfer the collateral into a single wallet address. (This wallet address will need to be the same address as the Master Node which you are upgrading into the Gateway Node, not the one which you are shutting down.)